Facebook has been accused of eavesdropping on supposedly private messages in order to keep track of user preferences and activity for targeted advertising.

Private messaging has become one of the killer apps of Facebook. While social networking is the company’s core service, direct exchange between users has also come to focus. Facebook’s release of its standalone Messenger app further cemented the importance of private messaging — a particularly important feat for Facebook given the rise of mobile as a preferred means of getting online.

The question now is how private really is Facebook’s messaging service. Facebook does not promise to provide fully-encrypted messaging like apps such as Wickr or Telegram. But Messenger’s terms of service is said to be insidious in that the Android app actually accesses user information (such as call logs, messages and access to the camera and microphone) without additional user consent, assuming one has given these permissions upon installing from Google Play.

Just before the new year, a couple of Facebook users have lodged a class action lawsuit, which alleges that the private messaging service is in violation of the Electronics Communication Privacy Act. The lawsuit accuses Facebook of intercepting “for purposes including but not limited to data mining and user profiling.”

According to Michael Sobol, an attorney for the plaintiffs, the tracking “is a mechanism for Facebook to surreptitiously gather data in an effort to improve its marketing algorithms and increase its ability to profit from data about Facebook users.”

The lawsuit is focusing on the alleged practice of scanning URLs embedded in private messages, which Facebook is said to be mining for user activity. The complaint specifies that it is the “private” representation of Facebook that causes undue trust from users, as they are made to believe that the messaging service is private, but is being tracked, after all.

Representing to users that the content of Facebook messages is ‘private’ creates an especially profitable opportunity for Facebook, because users who believe they are communicating on a service free from surveillance are likely to reveal facts about themselves that they would not reveal had they known the content was being monitored.”

The lawsuit, if it reaches class action, seeks $10,000 in damages for each member of the class. The complaint has been lodged on behalf of Facebook users in the US who have sent or received messages with a URL. A Facebook representative says the company believes “the allegations are without merit,” and says the company will defend itself “vigorously” against the accusation.